Ab initio investigation of cobalt siting in zeolites

Quantum chemical calculations offer an attractive methodology to study Co-exchanged zeolites. The reproduction of experimental spectroscopic signatures of cobalt before investigating the methane SCR cycle over these catalysts was described. Zeolite clusters were extracted from the ferrierite framework corresponding to sites B and G. The clusters contained ∼ 30 non-hydrogen atoms depending on the site. For each cobalt-ferrierite cluster, the spectroscopic signatures at the lowest energy geometry of the low and high spin states were computed to compare with experiment. For a given cluster, the low-spin Co-O bonds were slightly shorter than the high-spin Co-O bonds. In all cases, the high-spin state was preferred by 20-40 kJ/mole depending on the cluster, in agreement with experiment. The spectroscopic signatures correlated to the aluminum substitution position rather than the extraframework site. Accurate knowledge of cobalt extraframework siting was the necessary first step towards understanding cobalt-zeolite SCR catalysts. The interaction of cobalt in these sites with NO, O ₂, and NH ₃ molecules relevant to SCR catalysis was presented.